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WEEK FOUR
CARBOHYDRATES
The name carbohydrate is derived from the French ‘hydrate de carbone’ and was applied
to neutral chemical compounds containing the elements carbon, hydrogen and oxygen
with which hydrogen and oxygen occurring in the same proportion as in water (CH2O).
Chemically, carbohydrates are therefore polyhydroxyl aldehyde or ketones or substances
that yield these on hydrolysis. CHO consists of the greatest proportion of food consumed
by animals except the carnivores.
FUNCTIONS OF CARBOHYDRATE
1.
The primary function of carbohydrate in animal nutrition is to serve as a
source of energy for normal life processes.
2.
The relatively insoluble fractions (cellulose, hemicellulose) are most
important in providing structural support for living plants.
3.
Carbohydrates and lipids are the two major sources of energy for animal body
in normal conditions.
DIGESTION AND ABSORPTION OF CARBOHYDRATES
NON-RUMINANTS
Starch is the only polysaccharide which is highly utilized by monogastric animals and
dietary disaccharides have to be broken down by digestion into mono-sugars whose
molecules are suitable for passage through the intestinal tract. For absorption to occur,
poly, tri and disaccharides must be hydrolsed by digestive enzyme produced by the host
or the microflora inhabiting the GIT of the host. The following schemes illustrate the
mode of action of carbohydrates.
α-amylase
Maltose
α-glucosidase
1.
Starch
Glucose
2.
Saccharose
α-glucosidase
Glucose
+
fructose
3.
Lactose
β-Glucosidase
Glucose
+
Galactose
The major carbohydrate digesting enzyme is α-amylase secreted in the pancreatic juice
which splis the α-1,4- linkages. α-amlyase cannot hydrolyse α-1,6-branches or linkages.
Intra-cellular amylases complete the hydrolysis of starch started by pancreatic amylase in
the intestinal lumen. The mechanism of /or intestinal absorption of sugar involves active
transport. The rate of absorption of sugar decreases in the following order.
Galactose > Glucose > Fructose > Pentose
The absorbed sugars are carried by the portal blood to the liver.
Note: Cellulose and hemi-cellulose escaping the small intestine of poultry or pigs are
substrats of fermentation which occur in the caecum. This microbial action resemble the
ruminal fermentation of polysaccharide. The capacity of non-degraded CHO to absorb
water increases in bulkiness of the chime passing through the tract. This encourages a
peristaltic action by which food residues/digesta are drawn forward through the intestine
and mechanical digestion is enhanced.
RUMINANTS
There are fundamental differences between ruminant and mongastric animals concerning
the mode of digestion and metabolic pathways and the type of the end products formed
by the 2 groups of animals. The major bulk of carbohydrate in ruminants feed are
polymers; cellulose, hemicellulose, starch, fructose and pectin. Fodder plants contain on
dry matter bases 20 – 30% of cellulose, 14 – 20% hemicelluloses, up to 10% of pectin
and 2- 12% of lignin. The breakdown of carbohydrate in the rumen may be divided into 2
stages;
a.
First, is the digestion of complex carbohydrate to simple sugars. This is
brought about by extra-cellular microbial enzymes and it is thus analogous to
the digestion of carbohydrate in non-ruminants. Cellulose is decomposed by
one or more β-1,3 glucosidase to cellobiose which is then converted either to
glucose or through the action of a phosphorylase to glucose 1- phosphate.
Starch and dextrin are first converted to maltose, by maltose phosphorylase or
1-6-glucosidase to glucose 1- phosphate.
b.
Fructans are hydrolsed by enzymes attacking 2,1 and 2,6 linkages to give
fructose which may also be produced together with glucose by the digestion of
sucrose. Pentoses are the major products of hemicellulose breakdown which is
brought about by enzymes attacking β-1,4 linkages to give xylose and uronic
acids.
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Note: The cellulose in roughages are practically unutilisable by human beings and most
species of monogastric animals. This may however be well utilized by ruminants due to
ability of microorganisms that break it down.
Fermentation in the rumen is by far the most important means whereby ruminant animals
digest and utilize carbohydrates.
METABOLISM OF CARBOHYDRATE IN NON-RUMINANTS
Metabolism of carbohydrate is important to farm animals since carbohydrates in the body
are essential sources of energy in the body and the starting materials for biosynthesis of
fats and non essential amino acids. The major product of carbohydrate digestion in nonruminant is GLUCOSE and it is the starting materials for biosynthetic processes. The
central transporting medium for glucose is the blood. The blood glucose concentration is
determined by two opposing processes;
1.
Entry of glucose into the blood from the intestine, (originating from food)
liver and other organs.
2.
Withdrawal of glucose from blood into various tissues (liver, muscles, kidney,
adipose tissues and brain) and its utilization in the tissues for oxidation and
biosynthetic purposes.
The blood sugar is maintained by conversion of circulating blood glucose into glycogen
(glycogenesis) and re-conversion of glycogen to glucose (glycogenolysis). Hypoglycemia
is the condition of low level of glucose in the blood.
Hyperglycemia is the condition of high level of glucose in the blood.
SOURCES OF BLOOD GLUCOSE
1.
Absorption of glucose resulting from digestion of oligosaccharides and
polysaccharides.
2.
Biosynthetic formation in the body tissues particularly in the liver from noncarbohydrate metabolites, the such as amino acid, lactic acid, propionic acid
and glycerol (gluconeogenesis).
Note: that glucose is the second important energy source for ruminants after
volatile fatty acid (VFA) and it is formed mainly from biosynthetic processes
in these animals.
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3.
Glycogen stored in the liver serves as a reserviour for glucose when the latter
is needed in metabolic processes. The glucose is released from glycogen by
enzymatic breakdown of glycogenolysis. The glycogen stored are derived
from absorbed glucose or from glucose formed by gluconeogenesis.
The fate of glucose removed from the blood - the glucose removed from the
blood into cells and organs may be utilized in the following ways:
1. For the biosynthesis of glycogen.
2. For conversion into fat.
3. Conversion into amino acids.
4. It can also be used as a source of energy.
EMBDEM MAYERHOFS GLYCOLYTIC PATHWAY
The major pathway whereby glucose from feed is metabolized to give energy has two
stages. The 1st stage is known as GLYCOLYSIS. This can occur under anaerobic
condition and result in the production of pyruvate/pyruvic acid. The metabolism of CHO
is effected with the aid of their phosphate derivates.
Glycolysis involves the breaking down of a unit or 1 mole of glucose to yield pyruvic
acid. Enzymes are released during the process. It is a process involving
PHOSPHORYLATION REACTIONS and by the end of the pathway, a six carbon
molecule would have been successfully broken down into 2 moles of 3-carbon units. All
the steps of glycolysis are enzymatically catalysed.
Glycogen is stored in the liver and situation may arise that the glucose that is circulating
in the blood is depleted; for example in athletes or actively running animal, the glycogen
can be mobilized to release glucose through the process of GLYCONEOLYSIS.
The glucose can also be from the breakdown of starch in the diet. When glycogen is
broken down, it releases glucose unit as GLUCOSE-1-PHOSPHATE. Anytime a
carbohydrate is been phosphorylated, it requires a high energy constituent called ATP,
the glucose-1-phosphate is converted to glucose-6-phosphate by enzyme called
phosphoglucomutase. Most of the reactions are irreversible but the interconversion of
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glucose to glucose-6-phosphate and that of mannose to mannose-6-phosphate are
reversible.
Glucose-6-phosphate is converted by a reversible step to fructose-6-phosphate. The
enzyme responsible is phosphohexoisomerase. If fructose is present in the blood,
glucose-6-phosphate is produced directly from fructose and the enzyme responsible is
fructokinase.
Fructose-1,6 Biphosphate enzyme is phosphofructose kinase, this then breaks down to
form a 3 carbon unit called Triphosphate. This reaction is catalysed by Triosphosphateisomerate.
At this stage, there is a link between carbohydrate and fat metabolism because the
dihydroxyl acetone phosphate produced can be channeled to produce glycerol which is
important in fat metabolism.
3-phosphoglyceraldehyde is converted to 1,3 biphosphoglycerate, this need an energy
compound called NAD (nicotinic adenine dinucleotide).
The 1,3 biphosphoglyceric acids (PGA) formed is catalysed by phosphoglycerate mutase
because the phosphate group changes position from carbon 3 to carbon 2. After this, a
water molecule is removed to form phosphopyruvic acid (enol). The double bond so
formed here, is as a result of removal of water molecule, ADP and enzyme.
The next step is a spontaneous reversible reaction which form pyruvic acid or (ketone
form) which now goes on to form acetaldehyde. The enol form of pyruvic acid is not
stable because of the double-bond position because of its instability, it spontaneously
form keto-pyruvic acid.
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Note:
1. The 2 forms of pyruvic acid are (a) Enol form (production of an enol pyruvate)
(b) Ketone
2. The end product of gylcolysis is usually pyruvate.
COOH
COOH
C---OH
C=O
CH2
CH3
Pyruvic acid
Ketopyruvic acid
Acetaldehyde goes on to form an ethyl-alcohol by enzyme called Alcoholdehydrogenase
because the same enzyme facilitates forward and backward reactions. Pyruvic acid can go
into lactic acid in the presence of lactic acid dehydrogenases, this reaction occurs in
anaerobic condition where there is no oxygen. Under aerobic condition, pyruvic is
oxidized to CO2 and without producing energy.
Note: PPP i.e (Pentose Phosphate Pathway) is an alternative pathway to glycolysis and it
takes place in small animals (anaerobic condition).
FUNCTIONS OF METABOLIC PATHWAY
1.
Production of fuel and energy
2.
Conversion of glycogen
3.
Providing/Reducing ion e.g hydrogen ion for synthesis of fat.
4.
PPP –Pentose Phosphate Pathway (5 carbon sugar operate in PPP
environment) provides ribose sugar for synthesis of RNA and DNA.
5.
Triosphosphate is used for the synthesis of glycerol.
6.
production of pyruvate/pyruvic acid.
7.
All the intermediate substances in the TCA are used for protein synthesis.
8.
A-coA (Acetic-co-enzyme A) is used for the synthesis of fat and also
synthesis of cholesterol for steroids.
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